Elucidating Degradation Phenomena During Mixing of Silica-Natural Rubber Compounds: The Interplay of Viscoelastic Behavior and Silane Microstructures

Various commercial silica-silane coupling agent combinations are used these days for synthetic rubber-based passenger car tire treads with high silica-loadings. For truck tire treads, much lower silica-loadings are employed in combination with Natural Rubber (NR). The present study examines the impact of various commercial silane coupling agents on degradation phenomena during mixing of silica-filled NR compounds by analyzing changes in viscoelastic response, with a focus on different silane microstructures. A novel approach using delta–delta (Δδ) values to quantify branch formation caused by degradation-induced chain modifications has been adopted. Unfilled NR compounds primarily undergo polymer chain scission from thermo-mechanical and thermo-oxidative influences. During mixing, bis-(3-TriEthoxySilylPropyl) Tetrasulfide (TESPT) silane creates silica-rubber coupling and lightly crosslinked materials partly based on polysulfidic bonds. The lightly crosslinked rubber essentially sustains the reinforcement level in the compounds, counterbalancing the degradation effect. However, predominant degradation at 180°C significantly deteriorates the mechanical and dynamic properties of compounds containing both TESPT and TESPD (bis-(3-Triethoxysilylpropyl) disulfide). OTPTES (3-OctanoylThioPropylTriEthoxySilane) and MTCO (Mercapto-ThioCarboxylate Oligomer) silane-based compounds exhibit notable plasticization effects along with degradation. The choice of OTPTES in particular demonstrates excellent thermal stability, preserving mechanical properties even till 180°C. MTCO, with one reactive mercaptan group, couples efficiently with rubber at low dump temperatures, 130°C and 150°C, enhancing vulcanizate properties. At high dump temperatures of 170°C and 180°C, though, MTCO releases plasticizing moieties during mixing, leading to inferior final properties. The present study highlights the intricate balance between rubber degradation, crosslinking, branching, network formation, rubber-filler interactions, and plasticization effects specifically in silica-filled NR compounds for truck tire tread applications.